1. Field of the Invention
The present invention relates to an optical apparatus equipped with an aperture stop unit (light amount adjusting unit), such as a lens apparatus and an image pickup apparatus.
2. Description of the Related Art
Light amount adjusting units, generally called aperture stop units, move plural stop blades in opening and closing directions by an actuator such as a stepping motor to change the size of a stop aperture formed by the stop blades so as to increase and decrease an amount of light passing through the stop aperture.
When performing video capturing with operation of such an aperture stop unit, it is necessary to always keep smooth movement of the stop blades in response to variations of luminance of a captured object so as to ensure high aperture value accuracy. However, such smooth movement of the stop blades may be prevented due to backlash included in an opening/closing mechanism transmitting a driving force of the actuator to the stop blades to move them in the opening and closing directions, and due to a cogging torque of the stepping motor.
Japanese Patent No. 3535603 discloses an optical apparatus that has a lens drive mechanism transmitting a driving force of an actuator to a lens and controls the actuator so as to move the lens by a sum of a normal movement amount of the lens and an additional movement amount so as to avoid deterioration of focusing accuracy due to backlash in the lens driving mechanism. Also in the aperture stop unit, controlling the actuator so as to move the stop blades by a sum of a normal movement amount thereof and such an additional movement can reduce influences of the backlash in the opening/closing mechanism and the cogging torque of the stepping motor, which enables stop blade drive capable of ensuring high aperture value accuracy.
However, in the aperture stop unit, overlap of the stop blades and warpage of each stop blade cause sliding resistance, and the sliding resistance varies depending on a position of the stop blades (that is, depending on an aperture value) and a moving direction thereof. For example, the sliding resistance caused by the warpage of the stop blade increases when the stop blades are moved from a small aperture state (narrowed state) toward an opening direction. Thus, it is necessary for the aperture stop unit to consider not only the backlash of the opening/closing mechanism and the cogging torque of the stepping motor, but also the position and moving direction of the stop blades.
In this respect, the optical apparatus disclosed in Japanese Patent No. 3535603 employs a fixed additional movement amount for avoiding the deterioration of focusing accuracy due to the backlash in the lens driving mechanism regardless of the position of the lens. Therefore, it is difficult in the aperture stop unit to keep the smooth movement of the stop blades only by addition of such an additional movement amount.